1. Field of the Invention
The present invention generally relates to a combiner and a miniaturized optical combiner module. More particularly, the present invention relates to a combiner and an optical combiner module having small size and a digital light projection system using the optical combiner module to shorten back focal length.
2. Description of Related Art
Conventionally, the key component of the digital light processing (DLP) projector is a semiconductor component controlled by two-bits pulse tuning, wherein the semiconductor component is generally called a digital micro-mirror device (DMD). The DMD may control a digital optical switch very rapidly and may control the light source very precisely. The image displaying method of DLP projector is different from the conventional liquid crystal projector (LCP). In the conventional LCP, the image is displayed by the transmitted light via a liquid crystal panel. However, in the DLP projector, the image is displayed by reflecting the light from the micro mirrors on the digital micro-mirror device (DMD). Therefore, the weight of the DLP projector can be reduced down to less than 2.5 kg, however, the weight of a conventional projector is generally up to 8 to 15 kg. In addition, the size of the DLP projector is less than that of the conventional projector. Moreover, the optical efficiency and resolution of the DLP projector are much better than that of the conventional projector. Since the digital micro-mirror device (DMD) has high reflectivity and high fill factor, the optical efficiency of the digital micro-mirror device (DMD) is high. Therefore, the DLP projector is very suitable for the application requiring high brightness and high resolution. Moreover, the DLP projector may provides fully digitized color display, and precise and stable image display.
FIG. 1 is a plan diagram schematically illustrating the internal structure of a conventional digital processing projection device. Referring to FIG. 1, a conventional DLP projector is generally constructed by a light source 111, a rod integrator 112, an aspheric lens set 113, a reflection mirror 115, a beam splitter module 120 and a projection lens. The light 11 emitted by the light source 111 is propagated through the rod integrator 112 and aspheric lens set 113 and condensed in the reflection mirror 115.
Referring to FIG. 1, a conventional beam splitter and optical combiner module 120 includes a total internal reflection (TIR) prism 117 and a Philips prism 121. The light 11 described above is reflected by the reflection mirror 115 to the total reflection plane 117a of the total internal reflection (TIR) prism 117. It is noted that the incident angle A1 of the light 11 incident on the total reflection plane 117a is larger than the critical angle of total reflection. In addition, there is an air gap between prisms 31 and 32 of the total internal reflection (TIR) prism 117. Therefore, the light 11 is propagated from optically denser medium to optically less denser medium, and thus the light 11 is totally reflected on the total reflection plane 117a and is incident to the Philips prism 121.
There are two layers of coating in the Philips prism. For the coating 118a, the red light 13 of the light 11 is reflected, and the remainder color lights are transmitted. For the coating 118b, the blue light 15 of the remained color lights transmitted from the coating 118a is reflected, and thus the remainder green light 17 is transmitted. Accordingly, after the light 11 propagated through the Philips prism 121, the light 11 is split into the red light 13, the green light 17 and the blue light 15. Each of the three color lights are incident to digital micro-mirror devices (DMD) 121, 122 and 123 by a specific angle respectively.
The red light 13, green light 17 and blue light 15 incident on the digital micro-mirror devices (DMD) 121, 122 and 123 are reflected respectively, and the reflected red light 13, green light 17 and blue light 15 represent the red image, green image and blue image respectively. Thereafter, the red light 13, green light 17 and blue light 15 reflected by the digital micro-mirror devices (DMD) 121, 122 and 123 are incident on the total reflection plane 117a. In the meanwhile, the incident angle A2 of the red light 13, green light 17 and blue light 15 incident on the total reflection plane 117a is less than the critical angle of the total reflection of that. Therefore, the red light 13, the green light 17 and the blue light 15 are transmitted through the total reflection plane 117a of the total internal reflection (TIR) prism 117. Therefore, the red light 13, green light 17 and blue light 15 are projected via the projection lens 119.
In a conventional DLP projector, before the light is incident on the Philips prism, it is not split into the red light, the green light and the blue light. The color splitting of the light is performed by the Philips prism. The Philips prism at least has the disadvantages of heavy weight and large size. The Philips prism is the heaviest component of the whole DLP projector. In addition, the back focal length of the Philips prism is long. Furthermore, since the color splitting and combining of the light are all performed in the Philips prism, the thermal problem is usually generated and the projection quality is adversely influenced.